An offshore wind farm or wind park generally comprises a plurality of wind turbines that generate electricity. In order to transport the electricity to a utility grid, the electricity is first fed into an offshore substation for conversion to a suitable current/voltage level and then transported by cable to an onshore substation, where the electricity may undergo further voltage/current conversion before being fed into a grid. Electricity can be transported as alternating current (AC), but AC transport is generally associated with losses, particularly over long distances. Since the trend is to more offshore wind farms with increasing capacity, the electricity from such wind farms is preferably transported over a high-voltage direct current (HVDC) transmission link, generally by a subsea cable laid on the seabed. To this end, the three-phase AC electricity from the wind turbines is converted by an offshore converter at the offshore substation into high-voltage DC electricity for transport.
For various reasons, wind turbines of a wind farm may need to be shut down for some time, after which they must be started up again. To be able to re-start a wind turbine after a wind farm downtime, an external power source is necessary to keep various systems and components supplied with power during a downtime. For example, various motors and drives of the wind turbines need to be periodically re-started for brief intervals during the downtime (to prevent the various bearings from seizing, for instance). This can be achieved by realising the offshore AC/DC converter as a bi-directional power converter, so that some auxiliary power could be fed from the onshore converter (which is generally realised as a full bridge converter) back to the wind park over the HVDC transmission line. However, such a bi-directional offshore power converter is expensive and adds considerably to the cost of the wind park. As an alternative, the offshore converter can be realised as a one-directional converter, and an additional AC link may be laid parallel to the HVDC link, so that auxiliary power can be provided to the windfarm whenever necessary. Clearly, such an additional transmission link also adds considerably to the overall cost of the windfarm. Since future HVDC-connected wind farms will often be located 100-200 km from the shore, such an additional AC subsea cable connection presents a severe challenge economically as well as technically.